Have you actually contaced Zyzzx for their recommendation? They've been autoxing this setup for years now and I'm sure they have very specific recommendations for you application. No sense reinventing the wheel if they've done the homework.
regardless of whether or not you are using a tender or just a main spring and helper, stroke will be taken up by the weight of the car. Just because you have a tender on there doesn't mean you will magically keep the car from compressing the damper. How stiff your overall spring rate is will determine how much travel you will lose from just the weight of the car, and naturally, what ride height you set the car at.